An embryo goes from a single cell to a complex organism containing thousands of cells of multiple types. These cells assemble into tissues and organs with a precise architecture. The complexity of this architecture is at its most extreme in the group of animals that includes humans, the vertebrates. Research proposed here will use one of the closest invertebrate relatives of the vertebrates, the ascidian. Ascidians are primitive marine chordates that have the same basic architecture, or body plan, as the vertebrates, but as embryos have much fewer cells, and are under the control of a much smaller genome. Experiments proposed here will exploit developmental mutants to investigate two processes: the morphogenesis of the notochord and the patterning of the non-neural ectoderm. The notochord is one of the defining characteristics of the chordates, and the notochord is an important experimental paradigm for understanding tissue morphogenesis, specific experiments will focus on the role of a laminin gene product in organizing and directing the cells of the notochord to form a coherent tissue. In the absence of this gene product the notochord cells wander endlessly through the body. Experiments will explore how the laminin gene product interacts with previously characterized cell-signaling pathways known to be important in directing notochord cell motility. A second type of mutant that will be investigated disrupts the patterning of the trunk ectoderm of the ascidian larva. In this mutant various malformations are evident, the most interesting being the lack-of specific ectodermal thickenings that appear to be similar to vertebrate placodes. In the absence of these placode-like structures, the development of other tissues, in particular the pharyngeal endoderm, is abnormal. Specific aims are targeted at identifying the gene responsible for generating this phenotype, and understanding its normal function.